In large radio transmitters, several cooperating high frequency power transistors, are utilised, e.g. LDMOS. A matching procedure is required to make these power transistors work optimally by dividing the transmitted power equal between the transistors and to make the transmitter optimally linear. The matching procedure should compensate for any spread between the different transistors. A part of this matching procedure is to tune, and set, the same working point for the cooperating high frequency power transistors.
It is beneficial to automate this matching procedure. This can be done by a circuit, which measures the drain current at the MOS-transistor, or collector current if the transistor is a bi-polar transistor, while changing the gate voltage at the MOS-transistor, or base voltage in the case of a bi-polar transistor until a suitable drain current, or collector current, is achieved.
In the following description only the terms, drain and gate will be used. It is however clear to the man skilled in the art that a bipolar transistor may instead be used and in which case the term drain should be substituted with the term collector and the term gate should be substituted with the term base.
The drain current is normally measured with a resistor serially connected to the drain connection. Occasionally a common resistor is used for several transistors, in which case the transistors are turned on by one, while the others are turned off.
The most common praxis is that no mechanical relays should be present to disconnect the transistor after the matching procedure, which means that the resistor will still be present during the operation of the power transistors, i.e. during transmission by the transmitter. This resistor will then steal power from the transmission and it is thus important to have a resistor with very low resistance. Common values are ranging from 100 mΩ to only a few mΩ.
The voltage drop over this resistor, caused by the power idle current of the transistors, should then be measured with enough accuracy. That is, a voltage drop of only few millivolts should be measured with enough accuracy, requiring a measurement precision of parts of a millivolt.
In addition thereto the resistor, over which, this small voltage drop should be measured, is connected to the feeding voltage, maybe as high as 30 Volt.
A common differential amplifier stage in an integrated circuit has an offset voltage of a few millivolts and is thus not accurate enough to be used for this type of voltage measurement. A circuit could of course be designed, but it would require trimming to achieve the required accuracy. This is a drawback since one would prefer circuits, which easily can be mass-produced and which do not risk problems regarding aging and temperature dependence.
A chopper-stabilised amplifier could, however, fulfill the requirements.
If a monolith solution is required, i.e. a single integrated circuit, and the output of the matching function should be on the low side of the feeding voltage, a relatively high-voltage process needs to be employed, that is a process allowing designs having high feeding voltage. This could for instance be a bi-polar process using PNP-transistors. With PNP-transistors is however a number of problems associated.
The problems that need to be overcome are, amongst others, that the PNP-transistors are non-linear, have a saturation voltage, consume bas current, and works equally well in reverse mode i.e. with emitter as collector and vice verse. Further problems are that the transistors generate substrate current, and have a high base-collector capacitance.
Implemented as high-voltage bi-polar lateral PNP-transistors, the transistors are limited in working frequency. This is a problem since the subsequent filter in the chopper-stabilised amplifier would have to have a low cut-frequency, which in turn makes the complete system slow and that the subsequent filter components take a relatively large area on the chip.
These problems have, in the prior art, limited the use of PNP-transistors for a chopper function.